Serum urea concentrations were measured in Large White pigs from lines divergently selected for components of efficient lean growth rate and performance tested over three 14-d test periods starting at 30, 50, and 75 kg. Two methods of performance testing were used. Phase-fed pigs were fed to appetite isoenergetic diets differing in total lysine:energy ratio (0.58, 0.69, 0.81, 0.91, 1.01, 1.12, and 1.23 g/MJ of digestible energy), whereas diet-choice pigs were offered a choice of the 0.69 and 1.12 lysine:energy diets. Between test periods, all animals were fed one diet: 0.91 g of lysine/MJ of digestible energy. The study consisted of 230 boars and gilts with 150 pigs performance tested on phase-feeding and 80 pigs on diet-choice. The line selected for high lean food conversion had lower urea concentrations on each diet than the line selected for high lean growth rate, despite similar predicted lysine balances. Efficiency of lean growth rather than the rate of lean growth may be a better selection strategy in the context of nitrogen excretion. Urea concentrations at the end of each test period were correlated with lysine intake (0.33, 0.48 and 0.65; standard error, 0.08) and predicted lysine balance (0.39,0.44, and 0.64), but were uncorrelated with predicted lysine for protein deposition (0.01, 0.08, and 0.08) and maintenance. Urea concentration at the end of a test period was not a useful predictor of protein deposition, even after accounting for pretest variation in urea concentration and food intake during test. The expected response pattern of serum urea concentration to diets differing in total lysine:energy would be nonlinear, with the point of inflection occurring at the required dietary total lysine:energy for each genotype. However, there was no evidence of such an inflection point such that the prediction of lysine requirement from urea concentration was not possible for the selection lines in the study.

Leptin is synthesised and secreted from adipocytes into the blood stream and transported to the brain, where it acts to cause a release of factors which can reduce food intake (Houseknecht et al., 1998). There are two murine mutations of the recessive gene coding for leptin which are associated with obesity. The Lepob allele determines synthesis and secretion of leptin, while the Lepdb allele determines responsiveness to leptin. In the Edinburgh lean growth experiment in pigs, selection for high and low daily food intake (DFI) has been practiced for seven generations in a Large White herd, which provides the experimental resource to determine if the correlated response in fat deposition is consistent with insufficient leptin production or with insensitivity to leptin.

Summary Divergent selection over seven generations for daily food intake (DFI) and for two lean growth rate (LGA and LGS) selection objectives in pigs produced correlated responses in serum insulin‐like growth factor‐1 (IGF‐1) concentration. The LGA and LGS selection objectives were for lean growth, but with performance testing on ad libitum (LGA) or on restricted (LGS) feeding regimes. There were 342 animals in the study, with an average of 46 pigs in each selection line and 72 pigs in the control line. All animals were performance‐tested from 30 to 90 kg on an ad libitum feeding regime. Selection on DFI increased IGF‐1 at 6 weeks of age (158 versus 104 μg/l, SED 21; control 136 μg/l) and selection for high LGA increased IGF‐1 at 30 kg liveweight (202 versus 161 μg/l, SED 14; control 185 μg/l). At 6 weeks of age and at 30 kg of weight, there was no response in IGF‐1 with selection on LGS (128 versus 129 and 167 versus 153 μg/l, respectively) suggesting that IGF‐1 may not be a reliable physiological predictor of genetic merit for such a selection objective, which combined rate and efficiency of lean growth. IGF‐1 measured immediately prior to the start of performance test may be useful in determining which animals to performance test for a lean growth selection objective, whereas IGF‐1 measured at 6 weeks of age may be a useful physiological predictor of genetic merit for daily food intake during performance test.

Selection for high or low daily food intake (DFI) in Large White pigs resulted in higher serum leptin concentration, fat deposition and food intake in the high DFI line. The response in serum leptin concentration indicated that the higher fat deposition of the high DFI line was not due to insufficient leptin production, as in the Lepob/Lepob mouse. Serum leptin was more highly correlated with fat deposition than with food intake indicating that the response in serum leptin was primarily due to increased fat deposition rather than to higher energy intake per se. The low correlations between serum leptin measured at 30 kg and performance test traits indicate that serum leptin would not be efficient for selection of animals prior to performance test. However, the consistent positive correlations between serum leptin and a measure of fat deposition suggest that serum leptin could usefully be incorporated in selection criteria for genetic improvement of carcass lean content in pigs.

No abstract